Scanning tunneling microscope stages are generally adapted to locate a sample under the microscope needle. Since the tolerances for accurately locating samples are extremely small, the stage location is generally adjustable in both fine and coarse increments. Fine adjustments are used to position the sample precisely with respect to the scanning needle, such that the region of the sample to be scanned is located a predetermined distance from the needle and directly thereunder. The coarse adjustment mechanism is designed to allow access to the sample. Once the sample has been placed on the STM stage, the coarse adjustment mechanism moves the stage to a location where fine adjustment may be used to accurately position the sample under the needle. Fine adjustment is usually accomplished by adjusting the needle position.
Previous stage designs for the scanning tunneling microscope have relied on piezoelectric or magnetic stepping devices for coarse adjustment. The article, "An Electromagnetic Microscopic Positioning Device for the Scanning Tunneling Microscope," by Corb et al., J. Appl Phys., 58 (11), December 1985, describes a magnetic coarse positioning apparatus. Designs which have relied on piezoelectric or magnetic stepping devices, while providing the necessary mechanical rigidity, have provided only limited motion in just one or two dimensions. Other gross adjustment designs based upon mechanical motion utilize leaf spring, sliding apparatus or a combination of leaf spring and sliding apparatus. While providing the necessary mechanical rigidity, both of these arrangements have limitations. They provide only limited motion, they are unreliable, they are sensitive to environmental conditions (e.g., dust), and their movements generally lack repeatability.
In some prior art designs, for example, the apparatus described in Russian Patent Document No. 682751, position adjustment is accomplished using screw mechanisms to slide a support structure. Designs of this type are sensitive to environmental conditions. For example, dust will tend to cause non-linear and non-repeatable motions. Also, sliding apparatus tend to have backlash problems since the friction of the slide points will tend to prevent the stage portion from moving freely.
U.S. Pat. No. 4,635,887 describes an alternative stage adjustment mechanism. In this patent, the adjustment screws are mounted to move with a series of intermediate support structures to provide three dimensional movement to an external stage. The supporting walls for the flexible springs are floating. The final stage position is connected to the main stage (frame) through a long mechanical path including a series of bending members. This arrangement is disadvantageous because it reduces immunity from vibration, position accuracy and thermal stability.
IBM Technical Disclosure Bulletin, Vol. 15, No. 12, May 1973, "Frictionless X, Y, Z and Theta Micropositioning Table," by Aronstein and Holmstrom, describes a micropositioning table utilizing piezoelectric supports which are flexible in a first direction, while being rigid in a support direction 90 degrees perpendicular to the flexible direction. These supports are used to mount a moveable stage. The gross positioning achievable using this arrangement is limited in range because it is piezoelectric driven. In addition, it is susceptible to vibration due to the nature of the mounting. Finally, the separate bending supports are not referenced to a fixed frame.
In "An STM for Biological Applications: BIOSCOPE" by B. Michel and G. Travaglini, presented at the Third International Conference on Scanning Tunneling Microscopy, Oxford, U.K., July 4-8, 1988, three-dimensional coarse motion is achieved using a sliding stage which requires three precision machined and polished sapphire plates. The micrometers slide diagonally cut blocks with sapphire interfaces to obtain X, Y and Z motion. Again, the sliding motion makes this device sensitive to environmental conditions.
Since the invention of the scanning tunneling microscope (STM) by Binnig and Rohrer (See "Scanning Tunneling Microscopy," IBM J. Res. Develop., Vol. 30, pp. 355-369, 1986.), coarse positioning of the sample has remained an unresolved problem. The large size of conventional X-Y-Z stages and the three degrees of freedom required result in inherent thermal and mechanical instabilities. Thus measurements must compensate for vibrations and thermal drift. A disadvantage of coarse positioning systems which are limited to two directions (X and Y-directions) is poor access of the tip to the sample. Further, if positioning is performed with piezoelectric elements, direction and magnitude of the movements are unpredictable. Finally, because most coarse positioning systems are manually operated, they cannot be controlled from data acquisition software.
Another problem with conventional positioning systems is the failure to reference all movements directly to a fixed support structure. The adjusting mechanisms in all three dimensions should be referenced directly to a fixed support structure to achieve the accuracy required in STM applications.
Another problem involves the adaptation of STM technology to biological applications; up to now most applications of STM have dealt with small repetitive structures that could be found within the range of a piezoelectric fine positioner. In such applications an unreliable, rough positioning system was not a great disadvantage. However, in biological applications, where structures are much less dense or even unique, the chance of locating a particular feature within the range of the piezoelectric fine positioner is very low.
Thus it will be seen that an improved coarse adjustment mechanism would include a reliable apparatus for moving the stage in relatively large steps. Such a mechanism would further include means for accurately and repeatably moving the stage to within a predetermined distance of the STM needle, where the predetermined distance is sufficiently small to allow the fine adjustment mechanism to work. Such a mechanism would further include a means to connect the stage to a rigid base in order to reduce the transmission and amplification of vibrations. In addition, such an apparatus would include means to adapt the coarse adjustment to a mechanized adjustment means with a minimum of interconnect apparatus. Finally, such an apparatus would not include sliding means.
The present invention allows very precise mechanical X, Y and Z motion of the sample in a scanning tunneling microscope while maintaining the instrument's immunity to external vibrations. In addition, the present invention provides a relatively large range of motion in three dimensions.